Recent advances in the development of semiconductor and organic light-emitting diodes (LEDs and OLEDs) have made these solid-state devices suitable for use in general illumination applications, including architectural, entertainment, and roadway lighting, for example. As such, these devices are becoming increasingly competitive with light sources such as incandescent, fluorescent, and high-intensity discharge lamps.
A property used to characterize a light source is the correlated colour temperature (CCT) and there are a number of methods of controlling the CCT of an LED light source. For example, U.S. Pat. No. 6,411,046 discloses the calculation of colour temperature of light emitted by a luminaire with an array of multicoloured LEDs with at least one LED in each of a plurality of colours. The colour temperature is calculated based on ambient temperatures and preset values, and each set of coloured LEDs is driven to produce a desired colour temperature. U.S. Pat. No. 6,495,964 describes a method for controlling the colour temperature of white light through optical feedback. Measured light outputs are compared to desired outputs and each LED colour is driven accordingly to reach the desired output. This drive method illustrated in FIG. 1, includes a DC-to-DC fly-back converter along with a filtering capacitor and inductor. This configuration can be an efficient drive method, however it involves a large number of parts per LED.
U.S. Patent Application No. 2004/0036418 also discloses a drive method where a DC-to-DC converter is used to vary the current through several LED paths. A current switch and sensor is implemented to provide feedback and control to limit the current to defined levels as illustrated in FIG. 2. This method can be considered to be similar to a standard buck converter and provides an efficient way for controlling the current through a given LED string. This drive method however, does not provide effective drive control when multiple LED paths are employed to facilitate colour control. When two LED paths with different forward voltages are used, high side switches are used as current limiting devices. The function of current limiting using transistors as variable resistors can result in large losses which decreases the overall efficiency of the circuit.
In addition, shunting techniques can be used to provide variable current flow through the LEDs. For example, if the forward voltage across an LED within a string of LEDs changes, then the total forward voltage across the string will change by the forward voltage across that specific LED. Switching in this manner requires large inductors to smooth the large changes in forward voltage and current flow. In the absence of large inductors, power losses of significant magnitude will occur in the supply or in the drive circuitry. Drive methods that require large components due to heavy switching, which induces large power losses on the supply or drive circuitry, further do not lend themselves to miniaturization due to the size of these components.
In addition, light sources that use a phosphor coating to produce visible light are typically very sensitive to changes in their junction temperature. Changes in this junction temperature can cause shifts in the center wavelength of blue light, for example. Unfortunately, the excitation spectra of phosphors is typically configured such that the peak excitation wavelengths do not coincide with the center wavelength emitted by the LED, and therefore only minor shifts in the LED emission spectra can cause significant changes in the conversion efficiency of the phosphors. This configuration can produce significant changes in the CCT of the phosphor coated LEDs as they are dimmed or as the ambient temperature changes. These devices thus require additional methods of controlling their CCT. For example, International Patent Application Publication No. WO 03/024269 discloses a method of using amber LEDs in combination with “warm white” (low CCT) and “cool white” (high CCT) phosphor-coated LEDs to dynamically change the CCT of the white light they generate. This method however is limited to adjusting the colour temperature of phosphor coated white LEDs.
Furthermore, as an LED's junction temperature increases the relative luminous flux decreases as illustrated in FIG. 3 (Luxeon™ Emitter Technical Data Sheet DS25). If LEDs are driven at their rated power and the light output of a specific colour in the spectrum decreases, that colour of LED may have to be driven harder to compensate for this decrease. The increased current results in more heat, which may lead to an avalanche effect and permanent damage to the LEDs.
Therefore, there is a need for an apparatus and method of controlling the colour and colour temperature of light produced by a digitally controlled light source without significant power losses as well as circuits that have a small part count that can further enhance the efficiency of the circuit while maintaining a low overall system cost.
This background information is provided for the purpose of making known information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention.